The invention relates to a picture pickup device comprising a television camera tube. The camera tube has an electron gun in an evacuated envelope. The electron gun generates an electron beam. A photosensitive target, on which the beam is focused by focusing means and across which the beam is deflected by deflection means, is also arranged in the envelope. The deflected beam describes a line raster, on the target. The electron beam forms an elongated spot on the target. The longitudinal axis of the spot is perpendicular to the lines of the raster and has a length which is substantially equal to the line spacing.
The invention also relates to a television camera tube.
A picture pickup device and camera tube of this type are described in an article by M. Kurashige entitled "Formations of Elliptical Scanning Electron-Beam Cross Section for Removing Unscanned Area in High Resolution Camera Tubes" (Electronics and Communications). In Japan, Vol. 67-C, No. 2, pages 54-61 1984 (translated from Denshi Tsushin Gakkai Ronbunshi, Vol. 66-C, No. 9, Sept. 1983, pages 638-645). In this article it is proposed to remove electric charge stored in unscanned areas between the lines scanned on the target by giving the spot an elliptic shape or by rapidly moving a round spot up and down over a short distance with the aid of a radio-frequency electric field. The latter is referred to as wobbling. The elliptic spot is obtained with the aid of an elongated aperture in an electrode.
This method is applicable to a tube having magnetic focusing and deflection means, although the direction of the longitudinal axis of the spot changes slightly depending on the position of the spot on the target. This method is also applicable to an electrostatically focused tube with magnetic deflection. However, according to the kurashige article such a tube has a poor resolution. The direction of elongation of the spot is, however, independent of the position of the spot on the target. Wobbling may be used with tubes having magnetic focusing and electrostatic deflection.
The drawback of an elongated aperture in an electrode is that the extent of elongation of the spot can no longer be adjusted after the camera tube has been positioned in the deflection coils and/or focusing coil. Due to small differences between coils and small errors in the alignment of tube and coils with respect to each other, this adjustability is desirable. The drawback of wobbling in systems using magnetic deflection is that the high frequencies desired during wobbling generate interference in the deflection coils.
A disturbing phenomenon in pickup tubes is the stern-wave effect, which occurs particularly in tubes having a diode electron gun. This phenomenon occurs when a bright object (a bright portion of the picture) moves against a dark background. In that case a stern wave (a plurality of dark and light stripes decreasing in contrast and length) follows the object. It is not quite clear what these stern waves are caused by. It is, however, clear that they disappear when the spot is elongated and fills the space between two picture lines.